It has been recognized in the art that one solution to the barrier problem would be to place, between the metal and the wider band gap compound semiconductor, an intermediate smaller band gap semiconductor.
An early illustration of this is in U.S. Pat. No. 3,984,261 wherein a layer of indium gallium arsenide was employed as a contact to gallium arsenide (GaAs) along with alloying.
Further, in Electronics Letters 15, 24, 1979, p.800, a contact to GaAs is described having a lower band gap material where that lower band gap material Ge is grown on the GaAs. The Ge-GaAs lattices are closely matched. Low impedance is achieved by heavy doping and the fact that Ge is a dopant for GaAs. However, Ge as a low barrier semiconductor has a relatively large density of states in the conduction band.
A major advantage of the use of InAs as the intermediate low band gap material is that there is no significant barrier between the InAs and an external metal but as the art further developed, it became apparent that there was more than one barrier in series to be considered when a metal contact was made to a wider band gap semiconductor through an intermediate lower band gap semiconductor and that the barrier at the lower band gap/higher band gap interface for the material InAs would be a problem. A description of the barrier and a proposed solution by employing a graded band gap region is reported in J. Vac. Sci. Technol., 19 3, Sept./Oct. 1981, p.626; and a survey of the metal contact and interface problems where an intermediate lower band gap semiconductor region is used is reported in J. Vac. Sci. Technol., 19 3, Sept./Oct. 1981, p.794.
In Physical Review Letters, Vol. 51, No. 19, 7 Nov. 1983, p.1783, experimental further studies of the internal barrier reported that misfit disclocations in a planar array at an internal interface between GaInAs and GaAs were responsible for the barrier at higher band gap semiconductor interfaces.
It is becoming clear in the art that more than one barrier is involved where a low band gap semiconductor intermediate region between the metal and the higher band gap semiconductor is employed, that each can affect the overall performance of the contact and that each requires different considerations.